Motor-driven power steering system for a vehicle

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotative force of a motor.

2. Description of the Prior Art known to the Applicant.

[0002] In the past, a power steering system of this type, as schematically illustrated in Fig. 9, has hitherto been known to the Applicant in which a steering wheel 1 is operatively connected through a steering shaft 2 and a first rack and pinion gear T₁ including a first pinion 5 and a first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a, 8b so that when the steering wheel 1 is turned by an operator, the steerable road wheels (not shown) are appropriately steered in accordance with the steering motion of the steering wheel 1 imparted by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear including a second rack tooth portion 6b and a second pinion 18, a speed-reduction gear R, and a switching clutch 17 so that the driving force of the motor 13 is transmitted through the speed-reduction gear R, the switching clutch 17 and the second rack and pinion gear T₂ to the rack 6 so as to assist the steering operation of the steering wheel 1 imparted by the operator. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9 The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operation of the motor 13 and the switching clutch 17 on the basis of the steering torque and the vehicle speed measured.

[0003] The power steering system as constructed above has the following problems. Specifically, when the travelling speed of the vehicle increases above or decreases below a prescribed level, the control unit 9 operates to deenergize or energize the switching clutch 17 to interrupt or enable the transmission of the assisting force from the motor 13 to the rack 6 so as to switch the power steering system into a manual steering mode without any power assist or into a power steering mode with power assist. Accordingly, whenever the vehicle speed changes across a prescribed speed level, the operating mode of the steering system abruptly changes from power steering into manual steering or vice versa, thus causing the operator to feel a certain unsmoothness, unsureness, or instability in the steering.

[0004] In GB Patent Specification 2 161 770, a power steering system is described which comprises a single electromagnetic clutch which controls a coupling mechanism so that above a predetermined speed, the steering is manually controlled.

SUMMARY OF THE INVENTION

[0005] In view of the above, the present invention is intended to obviate the above-described problem of the prior art, known to the Applicant, and has for its object the provision of a novel and improved motor-driven power steering system for a vehicle of the kind described which can be switched or transferred from power steering into manual steering or vice versa in an extremely smooth manner without causing any unsure feelings.

[0006] According to the present invention, there is provided a motor-driven power steering system for a vehicle having a steering wheel operatively connected with steerable road wheels so that the steering wheel is turned by an operator to appropriately steer the steerable road wheels, the system comprising torque sensor means for providing a torque signal indicative of torque applied to the steering system by an operator, a motor for power-assisting the steering motion caused by the operator through the steering wheel, a control unit for providing a control signal (s) in dependence upon the torque signal and the speed of the vehicle, and switching clutch means decoupling the transmission of the power-assisting force in dependence upon the speed of the vehicle, characterised in that a further clutch means is provided for controlling transmission of power-assisting forces from the motor to the steerable road wheels, the further clutch means being controlled in dependence upon the control signal provided by the control unit.

[0007] Preferably, the power-assisting torque transmitted from the motor toward the rack through the further clutch is gradually decreased or increased in inverse proportion to the measured vehicle speed. In this case, such a power-assisting torque may be gradually decreased or increased in a stepwise fashion.

[0008] In one embodiment, the control unit comprises:

[0009] a steering-torque measuring means adapted to receive the output signal of the torque sensor for measuring the operator-induced steering torque;

[0010] a vehicle-speed measuring means adapted to receive the output signal of the vehicle-speed sensor for measuring the vehicle speed;

[0011] a memory means for storing current values to be supplied to the further clutch which are determined by the vehicle speed and the steering torque;

[0012] a clutch-current determining means adapted to read out from the memory means an appropriate current value corresponding to the measured steering torque and the measured vehicle speed for determining the current to be supplied to the further clutch; and

[0013] a clutch-current controlling means for controlling the current supplied to the further clutch in accordance with the output from the clutch-current determining means.

[0014] In this case, it is preferable that the memory means comprise;

[0015] a reduction-ratio storage means for storing second predetermined reduction ratios of the steering torque with respect to the vehicle speed; and

[0016] a control-torque calculating means for calculating an appropriate control torque by multiplying the measured steering torque by the corresponding reduction ratio, whereby the clutch-current determining means determines an appropriate level of clutch current based on the calculated control torque.

[0017] In another embodiment, the control unit comprises:

[0018] a steering-torque measuring means adapted to receive the output signal of the torque sensor for measuring the operator-induced steering torque;

[0019] a vehicle-speed measuring means adapted to receive the output signal of the vehicle-speed sensor for measuring the vehicle speed;

[0020] a memory means for storing voltage values to be imposed on the motor which are predetermined by the vehicle speed and the steering torque;

[0021] a motor-voltage determining means adapted to read out from the memory means an appropriate voltage value corresponding to the measured steering torque and the measured vehicle-speed for determining the voltage to be imposed on the motor; and

[0022] a motor-voltage controlling means for controlling the voltage imposed on the motor in accordance with the output of the motor-voltage determining means.

[0023] In this case, it is preferable that the memory means comprise;

[0024] a reduction-ratio storage means for storing predetermined reduction ratios of the steering torque with respect to the vehicle speed; and

[0025] a control-torque calculating means for calculating an appropriate control torque by multiplying the measured steering torque by the corresponding reduction ratio, whereby the motor-voltage determining means determines an appropriate level of motor voltage based on the calculated control torque.

[0026] In a further embodiment, the control unit comprises:

[0027] a steering-torque measuring means adapted to receive the output signal of the torque sensor for measuring the operator-induced steering torque;

[0028] a vehicle-speed measuring means adapted to receive the output signal of the vehicle-speed sensor for measuring the vehicle speed;

[0029] a first memory means for storing voltage values to be imposed on the motor which are predetermined by the vehicle speed and the steering torque;

[0030] a motor-voltage determining means adapted to read out from the first memory means an appropriate voltage value corresponding to the measured steering torque and the measured vehicle-speed for determining the voltage to be imposed on the motor; a motor-voltage controlling means for controlling the voltage imposed on the motor in accordance with the output of the motor-voltage determining means;

[0031] a second memory means for storing current values to be supplied to the further clutch which are predetermined by the vehicle speed and the steering torque;

[0032] a clutch-current determining means adapted to read out from the second memory means an appropriate current value corresponding to the measured steering torque and the measured vehicle-speed for determining the current to be supplied to the further clutch; and

[0033] a clutch-current controlling means for controlling the current supplied to the further clutch in accordance with the output from the clutch-current determining means.

[0034] In this case, it is preferable that the first memory means comprise:

[0035] a first reduction-ratio storage means for storing first predetermined reduction ratios of the steering torque with respect to the vehicle speed; and

[0036] a first control-torque calculating means for calculating a first appropriate control torque by multiplying the measured steering torque by the corresponding first reduction ratio, whereby the motor-voltage determining means determines an appropriate level of motor voltage based on the first calculated control torque;

[0037] and that the second memory means comprise:

[0038] a second reduction-ratio storage means for storing second predetermined reduction ratios of the steering torque with respect to the vehicle speed; and

[0039] a second control-torque calculating means for calculating a second appropriate control torque by multiplying the measured steering torque by the corresponding second reduction ratio, whereby the clutch-current determining means determines an appropriate level of clutch current based on the second calculated control torque.

[0040] The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of several presently preferred embodiments of the invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figs. 1 through 8 relate to a motor-driven power steering system in accordance with one embodiment of the present invention, in which

[0042] Fig. 1 is a schematic view illustrating a general arrangement of the same;

[0043] Fig. 2 is a block diagram showing a control unit and its related parts;

[0044] Fig. 3 is a block diagram showing a motor-voltage storage means and a clutch-current storage means of the control unit;

[0045] Fig. 4 is a graphic representation showing current/torque characteristics of a first electromagnetic clutch;

[0046] Fig. 5 is a characteristic view showing a steering-torque/motor-voltage relationship and a steering-torque/first-clutch-current relationship when the vehicle is standing still;

[0047] Fig. 6 is a characteristic view showing a steering torque/first-clutch-current relationship when the vehicle speed changes;

[0048] Fig. 7 is a characteristic view showing a vehicle speed/first-clutch current relationship and a vehicle speed/second-clutch current relationship when the steering torque is constant;

[0049] Fig. 8 is a flow chart showing the control processes of the power steering system of Fig. 1 as controlled by the control unit illustrated in Fig. 2.

[0050] Fig. 9 is a schematic view illustrating a general arrangement of a motor-driven power steering system for a vehicle, known to the Applicant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] The present invention will now be described in detail with reference to a presently preferred embodiment thereof as illustrated in the accompanying drawings. In the following, the same parts or elements of the embodiments will be identified by the same reference numerals and characters as employed in Fig. 9.

[0052] Referring to the drawings and first to Fig. 1, there is shown a general arrangement of a motor-driven power steering system for a vehicle constructed in accordance with the present invention. The motor-driven power steering system illustrated includes a steering wheel 1; a steering shaft 2 fixedly connected at its upper end with the steering wheel for integral rotation therewith and including an upper section 2a and a lower section 2b joined with each other through a first universal joint 4a; a torque sensor 3 mounted on the steering shaft 2 for detecting turning force imparted to the steering wheel 1 by an operator and generating an electrical signal representative of the detected steering torque; a first pinion 5 operatively connected at its upper end with the lower end of the steering shaft 2 through a second universal joint 4b; a rack 6 operatively connected at its opposite ends through ball joints 7a, 7b with tie rods 8a, 8b which are in turn connected with a pair of steerable road wheels (not shown), the rack 6 having a first tooth portion 6a in mesh with the first pinion 5 and a second tooth portion 6b axially spaced from or otherwise continuous with the first tooth portion 6a; a vehicle-speed sensor 10 for detecting the travelling speed of the vehicle and generating an output signal representative of the detected vehicle speed; a battery 11 adapted to be mounted on the vehicle; a key or ignition switch 12; a second pinion 18 in meshing engagement with the second rack tooth portion 6b on the rack 6; a motor 13 operatively connected through a speed-reduction gear R with the second pinion 18 for power assisting the steering motion of the rack 6 induced by the operator, the motor being preferably in the form of a direct current motor having a shunt winding field or a magnetic field and adapted to be driven to rotate by the battery 11; a first electromagnetic clutch 14 interposed between the motor 13 and the second pinion 18 for transmitting power therebetween substantially in proportion to the intensity of current supplied thereto, the first clutch 14 being preferably in the form of an electromagnetic sliding clutch such as, for example, a powder clutch, a hysteresis clutch or the like and directly coupled, in the illustrated embodiment, with an output shaft of the motor 13; a second clutch 17 in the form of an electromagnetic switching clutch interposed between the motor 13 and the second pinion 18 for selectively establishing or disconnecting an operative connection therebetween; and a control unit 109 adapted to receive output signals from the torque sensor 3 and the vehicle-speed sensor 10 for controlling the operations of the motor 13 and the first and second clutches 14, 17 in a manner such that the power-assisting force transmitted from the motor 13 toward the steerable road wheels is gradually decreased before the second switching clutch 17 is switched off, and gradually increased after the second switching clutch 17 is switched on. In the illustrated embodiment, the speed-reduction gear R comprises a worm 15 operatively connected with an output shaft of the first sliding clutch 14, and a worm wheel 16 in mesh with the worm 15. The second electromagnetic switching clutch 17 serves to mechanically couple or disconnect the worm wheel 16 with the second pinion 18.

[0053] As diagrammaticaly illustrated in Fig. 2, the control unit 109 comprises: a steering-torque measuring means 109a adapted to receive the output signal of the torque sensor 3 for measuring the operator-induced steering torque; a vehicle-speed measuring means 109b adapted to receive the output signal of the the vehicle-speed sensor 10 for measuring the vehicle speed; a first memory means 109c for storing voltage values to be imposed on the motor 13 which are predetermined by the vehicle speed and the steering torque; a motor-voltage determining means 109d adapted to read out from the first memory means 109c an appropriate voltage value corresponding to the measured steering torque and the measured vehicle-speed for determining the voltage to be imposed on the motor 13; a second memory means 109e for storing current values to be supplied to the first electromagnetic sliding clutch 14 which are predetermined by the vehicle speed and the steering torque; a clutch-current determining means 109f adapted to read out from the second memory means 109e an appropriate current value corresponding to the measured steering torque and the measured vehicle-speed for determining the current to be supplied to the first electromagnetic sliding clutch 14; a motor-voltage controlling means 109g for controlling the voltage imposed on the motor 13 in accordance with the output of the motor-voltage determining means 109d; a clutch-current controlling means 109h for controlling the current supplied to the first electromagnetic sliding clutch 14 in accordance with the output from the clutch-current determining means 109f; and a clutch controlling means 109i for controlling the second electromagnetic switching clutch 17 in a manner such that the second clutch 17 is switched off when the vehicle speed increases above a prescribed level, and switched on when the vehicle speed decreases below the prescribed level.

[0054] As illustrated in Fig. 3, the first memory means 109c comprises: a first reduction-ratio storage means 209a for storing first predetermined reduction ratios of the steering torque with respect to the vehicle speed; and a first control-torque calculating means 209b for calculating a first appropriate control torque by multiplying the measured steering torque by the corresponding first reduction ratio, whereby the motor-voltage determining means 109d determines an appropriate level of motor voltage based on the first calculated control torque. Also, the second memory means 109e comprises: a second reduction-ratio storage means 209c for storing second predetermined reduction ratios of the steering torque with respect to the vehicle speed; and a second control-torque calculating means 209d for calculating a second appropriate control torque by multiplying the measured steering torque by the corresponding second reduction ratio, whereby the clutch-current determining means 109f determines an appropriate level of clutch current based on the second calculated control torque.

[0055] Now, the operation of the power steering system of this embodiment will be described with reference to Figs. 1 through 8. First, the case in which a vehicle is stationary or standing still will be considered. In this case, when the key switch 12 is first turned on to start the engine, the electromagnetic clutch 17 is automatically actuated to place the second pinion 18 into mechanical coupling with the worm wheel 16. In this state, when the steering wheel 1 is turned by an operator, the control unit 109 acts to control the operations of the motor 13 and the sliding clutch 14 in the manner as illustrated in Fig. 5 which shows a relationship between steering torque, motor voltage and clutch current. More specifically, when steering torque increases in the righthand direction to point a in Fig. 5, the motor 13 is turned on and then imposed with 100% voltage at point b. As the steering torque further increases, current begins to flow through the electromagnetic clutch 14 at point c and the intensity of the current increases logarithmically with respect to an increase in the steering torque, and reaches 100% current at point d. On the other hand, as the steering torque decreases, current flowing through the electromagnetic sliding clutch 14 begins to decrease at point d and reaches 0% current at point c. In accordance with a further decrease in the steering torque, the motor 13 is turned off at point e so that the voltage imposed on the motor 13 becomes 0% at point f. Similar to this, the motor 13 and the first electromagnetic sliding clutch 14 are controlled in the same manner when the steering torque increases or decreases in the lefthand direction.

[0056] As shown in Fig. 4, the characteristic of the sliding clutch 14 is used in a range in which transmitting torque or sliding torque increases substantially in direct proportion to clutch current. Accordingly, as is clear from Fig. 5, the motor 13 is imposed with 100% voltage and energized to start rotating when steering torque increases to point a. As the steering torque further increases, current flowing through the sliding clutch 14 begins to gradually increase at point c so that the output torque transmitted from the sliding clutch 14 to the worm 15 increases gradually. As a result, auxiliary torque having an intensity corresponding to the turning force imparted to the steering wheel by an operator is transmitted from the motor 13 to the second tooth portion 6b on the rack 6 via the sliding clutch 14, the worm wheel 16, the electromagnetic clutch 17 and the second pinion 18, thereby lightening the steering operation for the operator.

[0057] Now, the case in which the vehicle is travelling will be considered. In this case, as seen from Fig. 6, current flowing through the first sliding clutch 14 is controlled by the control unit 109 on the basis of the current value which is previously stored in the clutch-current storage means 109e and which is determined by the steering torque and the vehicle speed in an appropriate manner. Specifically, as shown in Fig. 6, the clutch current is determined such that it begins to rise or increase at point c and reaches 100% or full static current at point d, the rising curve becoming more gradual in accordance with an increase in the vehicle speed. Also, the starting point of the rising curve displaces from a smaller steering torque side toward a larger steering torque side in accordance with an increase in the vehicle speed. More particularly, in cases where the steering torque is assumed to be constant, the clutch current supplied to the first sliding clutch 14 is controlled to decrease in a stepwise fashion in inverse proportion to an increase in the vehicle speed. For example, the clutch current is controlled by the control torque determined by the product of the steering torque and a reduction ratio which corresponds to the vehicle speed so that it gradually decreases in a stepwise manner as illustrated in Fig. 7. Thus, the clutch current is made zero at point f even when steering torque is imparted to the steering wheel 1. When the vehicle speed further increases to point h in Fig. 7, the second electromagnetic switching clutch 17 is switched off or deenergized to release the engagement or mechanical coupling between the worm wheel 16 and the second pinion 18, thereby switching the steering system into a manual steering mode without any power assist. In this manner, the torque transmitted from the motor 13 to the rack 6 is gradually decreased in accordance with an increase in the vehicle speed under the action of the first sliding clutch 14 so that it is made zero before switching off or deenergization of the second switching clutch 17. On the other hand, as the vehicle speed decreases, the transmission torque or sliding torque of the first sliding clutch 14 begins to increase after the second switching clutch 17 is energized to mechanically couple the worm wheel 16 with the second pinion 18. As a result, the steering system can change over from the power steering mode into the manual steering mode or vice versa in an extremely smooth manner without giving rise to any feelings of unsureness or instability which would otherwise be caused by abrupt connection or disconnection of the second switching clutch 17.

[0058] Also, the voltage imposed on the motor 13 is likewise controlled in the same manner as the current supplied to the first sliding clutch 14. Though not illustrated, it is possible to control either the motor voltage or the clutch current supplied to the first sliding clutch 14 alone.

[0059] The above control of the power steering system can be installed simply by replacing the conventional control program of a control unit with that of the present invention without changing the general arrangement of a conventional power steering system. Accordingly, it is possible to manufacture the motor-driven power steering system of the present invention at low cost.

Claims

1. A motor-driven power steering system for a vehicle having a steering wheel operatively connected with steerable road wheels so that the steering wheel is turned by an operator to appropriately steer the steerable road wheels, the system comprising torque sensor means (3) for providing a torque signal indicative of torque applied to the steering system by an operator, a motor (13) for power-assisting the steering motion caused by the operator through the steering wheel, a control unit (9) for providing a control signals in dependence upon the torque signal and the speed of the vehicle, and switching clutch means (17) for coupling or decoupling the transmission of the power-assisting force in dependence upon the speed of the vehicle, characterised in that a further clutch means (14) is provided for controlling transmission of power-assisting forces from the motor (13) to the steerable road wheels, the further clutch means (14) being controlled in dependence upon the control signal (2) provided by the control unit (9).

2. A motor-driven power steering system according to claim 1, comprising a vehicle-speed sensor (10) for detecting the vehicle speed to generate an output signal representative of the detected vehicle speed, wherein:
the motor (13) is adapted to be energized by a battery (11);
the switching clutch means (17) is in the form of an electromagnetic switching clutch adapted to be switched off for interrupting the transmission of the power-assisting force from the motor (13) toward the steerable road wheels when the vehicle speed increases above a prescribed level, and switched on for transmission of the power-steering force therebetween when the vehicle speed decreases below the prescribed level; and
the further clutch means (14) is in the form of an electromagnetic sliding clutch for controlling the power-assisting force transmitted from the motor (13) to the steerable road wheels;
the control unit (109) is adapted to receive output signals from the torque sensor (3) and the vehicle-speed sensor (10) for controlling the operations of the motor (13) and the further clutch means (14) in a manner such that the power-steering force transmitted from the motor (13) to the steerable road wheels through the sliding clutch (14) is gradually decreased before the switching clutch means (17) is switched off, and gradually increased after the switching clutch means (17) is switched on.

3. A motor-driven power steering system for a vehicle as set forth in claim 2, wherein the power-assisting force transmitted from the motor (13) to the steerable road wheels through the sliding clutch (14) is gradually decreased or increased in inverse proportion to the vehicle speed.

4. A motor-driven power steering system for a vehicle as set forth in claim 3, wherein the power-assisting force transmitted from the motor (13) to the steerable road wheels through the sliding clutch (14) is gradually decreased or increased in a stepwise fashion.

5. A motor-driven power steering system for a vehicle as set forth in claim 2, wherein the control unit (109) comprises:
a steering-torque measuring means (109a) adapted for receiving the output signal of the torque sensor (3) for measuring the operator-induced steering torque;
a vehicle-speed measuring means (109b) adapted for receiving the output signal of the vehicle-speed sensor (10) for measuring the vehicle speed;
a memory means (109e) for storing current values to be supplied to the sliding clutch (14) which are predetermined by the vehicle speed and the steering torque;
a clutch-current determining means (109f) adapted for reading out from the memory means (109e) an appropriate current value corresponding to the measured steering torque and the measured vehicle-speed for determining the current to be supplied to the sliding clutch (14); and
a clutch-current controlling means (109h) for controlling the current supplied to the sliding clutch (14) in accordance with the output from the clutch-current determining means (109f).

6. A motor-driven power steering system for a vehicle as set forth in claim 2, wherein the control unit (10) comprises:
a steering-torque measuring means (109a) adapted for receiving the output signal of the torque sensor (3) for measuring the operator-induced steering torque;
a vehicle-speed measuring means (109b) adapted for receiving the output signal of the vehicle-speed sensor (10) for measuring the vehicle speed;
a memory means (109c) for storing voltage values to be imposed on the motor (13) which are predetermined by the vehicle speed and the steering torque;
a motor-voltage determining means (109d) adapted for reading out from the memory means (l09c) an appropriate voltage value corresponding to the measured steering torque and the measured vehicle-speed for determining the voltage to be imposed on the motor; and
a motor-voltage controlling means (109g) for controlling the voltage imposed on the motor in accordance with the output of the motor-voltage determining means (109d).

7. A motor-driven power steering system for a vehicle as set forth in claim 6, wherein the control unit (109) comprises:
a second memory means (109e) for storing current values to be supplied to the sliding clutch (14) which are predetermined by the vehicle speed and the steering torque;
a clutch-current determining means (109f) adapted for reading out from the second memory means (109e) an appropriate current value corresponding to the measured steering torque and the measured vehicle-speed for determining the current to be supplied to the sliding clutch; and
a clutch-current controlling means (109h) for controlling the current supplied to the sliding clutch (14) in accordance with the output from the clutch-current determining means.

8. A motor-driven power steering system for a vehicle as set forth in claim 5 or claim 6, wherein the memory means (109e) comprises:
a reduction-ratio storage means (209c) for storing predetermined reduction ratios of the steering torque with respect to the vehicle speed; and
a control-torque calculating means (209d) for calculating an appropriate control torque by multiplying the measured steering torque by the corresponding reduction ratio, whereby the clutch-current determining means (109f) determines an appropriate level of clutch current based on the calculated control torque.

9. A motor-driven power steering system for a vehicle as set forth in claim 7, wherein the first memory means (109c) comprises:
a first reduction-ratio storage means (109a) for storing first predetermined reduction ratios of the steering torque with respect to the vehicle speed; and
a first control-torque calculating means (109b) for calculating a first appropriate control torque by multiplying the measured steering torque by the corresponding first reduction ratio, whereby the motor-voltage determining means (109d) determines an appropriate level of motor voltage based on the first calculated control torque;
and wherein the second memory means (109e) comprises:
a second reduction-ratio storage means (209c) for storing second predetermined reduction ratios of the steering torque with respect to the vehicle speed; and
a second control-torque calculating means (209d) for calculating a second appropriate control torque by multiplying the measured steering torque by the corresponding second reduction ratio, whereby the clutch-current determining means (109f) determines an appropriate level of clutch current based on the second calculated control torque.

10. A motor-driven power steering system according to any one of claims 2 to 9, comprising:
a first pinion (5) operatively connected with a steering shaft (2) and adapted to be driven by the steering wheel (1) through the intermediary of the steering shaft;
a rack (6) operatively connected through tie rods (8a,8b) with a pair of steerable road wheels and having a first rack tooth portion (6a) and a second rack tooth portion formed (6b) thereon, the first rack tooth portion being in meshing engagement with the first pinion;
a second pinion (18) being in meshing engagement with the second rack tooth portion on said rack; wherein:
the motor is operatively connected through a speed-reduction gear (R) with the second pinion (18) for power assisting the steering motion of the rack induced by the operator through the intermediary of the steering wheel, the steering shaft (2), the first pinion (5) and the first rack tooth portion;
the sliding clutch (14) interposed between the motor and the second pinion (18) being operative for transmitting power therebetween substantially in proportion to the intensity of current supplied thereto; and
the switching clutch (17) interposed between the motor and the second pinion (18) being operative for selectively disconnecting the operative connection between the motor and the second pinion (18) when the vehicle speed increases above said prescribed level and establishing the operative connection therebetween when the vehicle speed decreases below said prescribed level.

Revendications

1. Système de direction assistée entraîné par moteur pour un véhicule ayant un volant de direction connecté de façon fonctionnelie à des roues directionnelles du véhicule, de sorte que le volant de direction est tourné par un opérateur pour braquer, de façon appropriée, les roues directionnelles du véhicule, le système comprenant un moyen de détection du couple (3) pour fournir un signal de couple indicateur du couple appliqué au système de direction par un opérateur, un moteur (13) pour assister en puissance le mouvement de braquage provoqué par l'opérateur à l'aide du volant de direction, une unité de contrôle (9) pour fournir des signaux de contrôle dépendant du signal de couple et de la vitesse du véhicule, et un moyen formant accouplement de commutation (17) pour coupler ou découpler la transmission de la force d'assistance de puissance en fonction de la vitesse du véhicule, caractérisé en ce qu'un autre moyen formant accouplement (14) est prévu pour contrôler la transmission des forces d'assistance en puissance depuis le moteur (13) vers les roues directionnelles du véhicule, l'autre moyen formant accouplement (14) étant contrôlé en fonction du signal de contrôle (2) délivré par l'unité de contrôle (9).

2. Système de direction assistée entraîné par moteur, en accord avec la revendication 1 , et comprenant un détecteur de vitesse du véhicule (10) pour détecter la vitesse du véhicule de manière à générer un signal de sortie représentatif de la vitesse détectée du véhicule, dans lequel :
le moteur (13) est adapté pour être alimenté par une batterie (11);
le moyen formant accouplement de commutation (17) a la forme d'un embrayage de commutation électromagnétique adapté pour être mis à l'arrêt pour interrompre la transmission de la force d'assistance en puissance provenant du moteur (13) vers les roues directionnelles du véhicule, lorsque la vitesse du véhicule augmente au-dessus d'un niveau prescrit, et pour commuter en marche la transmission de la force d'assistance en puissance entre ceux-ci lorsque la vitesse du véhicule diminue en dessous du niveau prescrit; et
l'autre moyen formant accouplement (14) a la forme d'un embrayage mobile électromagnétique pour contrôler la force d'assistance en puissance transmise depuis le moteur (13) vers les roues directionnelles du véhicule;
l'unité de contrôle (109) est adaptée pour recevoir des signaux de sortie provenant du détecteur de couple (3) et du détecteur de vitesse du véhicule (10), afin de contrôler le fonctionnement du moteur (13) et de l'autre moyen formant accouplement (14) de telle sorte que la force d'assistance au braquage transmise depuis le moteur (13) vers les roues directionnelles du véhicule, par l'intermédiaire de l'accouplement mobile (14), est graduellement diminuée avant que le moyen formant accouplement de commutation (17) ne soit désactivé, et est graduellement augmentée après que le moyen formant accouplement de commutation (17) soit activé.

3. Système de direction assistée entraîné par moteur pour un véhicule, tel que décrit dans la revendication 2, dans lequel la force d'assistance en puissance transmise depuis le moteur (13) vers les roues directionnelles du véhicule par l'intermédiaire de l'accouplement mobile (14) est graduellement diminuée ou augmentée en proportion inverse à la vitesse du véhicule.

4. Système de direction assistée entraîné par moteur pour un véhicule, tel que décrit dans la revendication 3, dans lequel la force d'assistance en puissance transmise depuis le moteur (13) vers les roues directionnelles du véhicule par l'intermédiaire de l' accouplement mobile (14) est graduellement diminuée ou augmentée palier par palier.

5. Système de direction assistée entraîné par moteur pour un véhicule, tel que décrit dans la revendication 2, dans lequel l'unité de contrôle (109) comprend :
un moyen de mesure du couple de braquage (109a) adapté pour recevoir le signal de sortie provenant du détecteur de couple (3) afin de mesurer le couple de braquage induit par l'opérateur;
un moyen de mesure de la vitesse du véhicule (109b) adapté pour recevoir le signal de sortie provenant du détecteur de vitesse du véhicule (10) afin de mesurer la vitesse du véhicule;
un moyen formant mémoire (109e) pour emmaganiser les valeurs du courant à appliquer à l'accouplement mobile (14) qui sont prédéterminées par la vitesse du véhicule et le couple de braquage;
un moyen de détermination du courant d' accouplement (109f) adapté pour lire dans le moyen formant mémoire (109e) une valeur de courant appropriée correspondant au couple de braquage mesuré et à la vitesse mesurée du véhicule, afin de déterminer le courant à fournir à l' accouplement mobile (14); et
un moyen de contrôle du courant d'accouplement (109h) afin de contrôler le courant fourni à l'accouplement coulissant (14) en accord avec la sortie du moyen de détermination du courant d'accouplement (109f).

6. Système de direction assistée entraîné par moteur pour un véhicule selon la revendication 2, dans lequel l'unité de contrôle (10) comprend :
un moyen de mesure du couple de braquage (109a) adapté pour recevoir le signal de sortie provenant du détecteur de couple (3) afin de mesurer le couple de braquage induit par l'opérateur;
un moyen de mesure de la vitesse du véhicule (109b) adapté pour recevoir le signal de sortie du détecteur de vitesse du véhicule (10) afin de mesurer la vitesse du véhicule;
un moyen formant mémoire (109c) pour emmagasiner des valeurs de tension à imposer au moteur (13) qui sont prédéterminées par la vitesse du véhicule et le couple de braquage;
un moyen de détermination de la tension du moteur (109d) adapté pour lire dans le moyen formant mémoire (109c) une valeur de tension appropriée qui correspond au couple de braquage mesuré ainsi qu'à la vitesse mesurée du véhicule pour déterminer la tension à imposer au moteur; et
un moyen de contrôle de la tension du moteur (109g) afin de contrôler la tension imposée au moteur en accord avec la sortie du moyen de détermination de la tension du moteur (109d).

7. Système de direction assistée entraîné par moteur pour un véhicule selon la revendication 6, dans lequel l'unité de contrôle (109) comprend :
un second moyen formant mémoire (109e) pour emmagasiner des valeurs de courant à fournir à l' accouplement mobile (14) qui sont prédéterminées par la vitesse du véhicule et le couple de braquage;
un moyen de détermination du courant d'accouplement (109f) adapté pour lire dans le second moyen formant mémoire (109e) une valeur appropriée de courant qui correspond au couple de braquage mesuré ainsi qu'à la vitesse mesurée du véhicule afin de déterminer le courant à fournir à l' accouplement coulissant ; et
un moyen de contrôle du courant d'accouplement (109h) pour contrôler le courant fourni à l' accouplement mobile (14) en accord avec la sortie du moyen de détermination du courant d' accouplement.

8. Système de direction assistée entraîné par moteur pour un véhicule selon la revendication 5 ou 6, dans lequel le moyen formant mémoire (109e) comprend :
un moyen d'emmagasinement de taux de réduction (209c) pour emmagasiner des taux de réduction prédéterminés du couple de braquage en fonction de la vitesse du véhicule; et
un moyen de calcul du couple de contrôle (209d) afin de calculer un couple de contrôle approprié en multipliant le couple de braquage mesuré par le taux de réduction correspondant, de sorte que le moyen de détermination du courant d'accouplement (102f) détermine un niveau approprié de courant d'accouplement sur la base du couple de contrôle calculé.

9. Système de direction assistée entraîné par moteur pour un véhicule selon la revendication 7, dans lequel le premier moyen formant mémoire (109c) comprend :
un premier moyen d'emmagasinage de taux de réduction (109a) pour emmagasiner un premier taux de réduction prédéterminé du couple de braquage en fonction de la vitesse du véhicule; et
un premier moyen de calcul du couple de contrôle (109b) afin de calculer un premier couple de contrôle approprié en multipliant le couple de braquage mesuré par le premier taux de réduction correspondant, de sorte que le moyen de détermination de la tension du moteur (109c) détermine un niveau approprié de tension du moteur sur la base du premier couple de contrôle calculé;
et dans lequel le second moyen formant mémoire (109e) comprend :
un second moyen d`emmagasinage de taux de réduction (209c) afin d'emmagasiner un second taux de réduction prédéterminé du couple de braquage en fonction de la vitesse du véhicule; et
un second moyen de calcul du couple de contrôle (209d) afin de calculer un second couple de contrôle approprié en multipliant le couple de braquage mesuré par le second taux de réduction correspondant, de sorte que le moyen de détermination du courant d'accouplement (109f) détermine un niveau approprié de courant d'accouplement sur la base du second couple de contrôle calculé.

10. Système de direction assistée entraîné par moteur en accord avec l'une quelconque des revendications 2 à 9, comprenant :
un premier pignon (5) connecté de façon fonctionnelle avec une colonne de direction (2) et adapté pour être entraîné par le volant de direction (1) par l'intermédiaire de la colonne de direction;
une crémaillère (6) connectée de façon fonctionnelle par l'intermédiaire de tiges de bielle (8a, 8b) avec une paire de roues directionnelles du véhicule et possédant une première portion dentée (6a) de crémaillère ainsi qu'une second portion dentée (6b) de crémaillère formée sur celle-ci, la première portion dentée de crémaillère étant en prise d'engrenage avec le premier pignon;
un second pignon (18) étant en prise d'engrenage avec la seconde portion dentée de crémaillère de ladite crémaillère; dans laquelle:
le moteur est connecté de façon fonctionnelle par l'intermédiaire d'un engrenage de réduction de vitesse (R) avec le second pignon (18) afin d'assister en puissance le mouvement de braquage de la crémaillère qui est induit par l'opérateur par l'intermédiaire du volant de direction, de la colonne de direction (2), du premier pignon (5) et de la première portion dentée de crémaillère;
l' accouplement mobile (14) interposé entre le moteur et le second pignon (18) fonctionnant pour transmettre la puissance entre ceux-ci substantiellement en proportion avec l'intensité du courant fourni à celui-ci; et
l' accouplement de commutation (17) interposé entre le moteur et le second pignon (18) fonctionnant pour sélectivement disconnecter le couplage fonctionnel entre le moteur et le second pignon (18) lorsque la vitesse du véhicule augmente au-dessus dudit niveau prescrit, et pour établir le couplage fonctionnel entre ceux-ci lorsque la vitesse du véhicule diminue en dessous du niveau prescrit.

Ansprüche

1. Motorgetriebenes Servolenksystem für ein Fahrzeug mit einem lenkrad, das betriebsmäßig mit lenkbaren Laufrädern so verbunden ist, daß es von einem Fahrer zum richtigen Lenken der lenkbaren Laufräder drehbar ist, wobei das System umfaßt: einen Drehmomentsensor (3), der ein ein vom Fahrer auf das Lenksystem aufgebrachtes Drehmoment anzeigendes Drehmomentsignal liefert, einen Motor (13), der die vom Fahrer durch das Lenkrad bewirkte Lenkbewegung mit einer Lenkhilfskraft unterstützt, eine Steuereinheit (109), die in Abhängigkeit vom Drehmomentsignal und der Fahrzeuggeschwindigkeit ein Steuersignal liefert, und eine Schaltkupplung (17), die die Übertragung der Lenkhilfskraft in Abhängigkeit von der Fahrzeuggeschwindigkeit ankoppelt oder abkoppelt, dadurch gekennzeichnet, daß eine weitere Kupplung (14) vorgesehen ist, die die Übertragung der Lenkhilfskräfte vom Motor (13) auf die lenkbaren Laufräder steuert, wobei die weitere Kupplung (14) in Abhängigkeit von dem von der Steuereinheit (109) gelieferten Steuersignal (2) gesteuert wird.

2. Motorgetriebenes Servolenksystem nach Anspruch 1, umfassend einen Fahrzeuggeschwindigkeitssensor (10), der die Fahrzeuggeschwindigkeit aufnimmt und ein der aufgenommenen Fahrzeuggeschwindigkeit entsprechendes Ausgangssignal erzeugt, wobei:
der Motor (13) von einer Batterie (11) aktivierbar ist;
die Schaltkupplung (17) als elektromagnetische Schaltkupplung ausgebildet ist, die ausschaltbar ist, um die Übertragung der Lenkhilfskraft vom Motor (13) auf die lenkbaren Laufräder zu unterbrechen, wenn die Fahrzeuggeschwindigkeit über einen vorbestimmten Wert ansteigt, und die zur Übertragung der Lenkhilfskraft dazwischen einschaltbar ist, wenn die Fahrzeuggeschwindigkeit unter den vorbestimmten Wert sinkt; und
die weitere Kupplung (14) als elektromagnetische Rutschkupplung ausgebildet ist, die die vom Motor (13) auf die lenkbaren Laufräder übertragene Lenkhilfskraft steuert;
die Steuereinheit (109) Ausgangssignale vom Drehmomentsensor (3) und vom Fahrzeuggeschwindigkeitssensor (10) empfängt und den Betrieb des Motors (13) und der weiteren Kupplung (14) derart steuert, daß die vom Motor (13) durch die Rutschkupplung (14) auf die lenkbaren Laufräder übertragene Lenkhilfskraft allmählich verringert wird, bevor die Schaltkupplung (17) ausgeschaltet ist, und allmählich erhöht wird, nachdem die Schaltkupplung (17) eingeschaltet ist.

3. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 2, wobei die vom Motor (13) durch die Rutschkupplung (14) auf die lenkbaren Laufräder übertragene Lenkhilfskraft im umgekehrten Verhältnis zur Fahrzeuggeschwindigkeit allmählich verringert oder erhöht wird.

4. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 3, wobei die vom Motor (13) durch die Rutschkupplung (14) auf die lenkbaren Laufräder übertragene Lenkhilfskraft stufenweise allmählich verringert oder erhöht wird.

5. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 2, wobei die Steuereinheit (109) umfaßt:
eine Lenkdrehmoment-Meßeinrichtung (109a), die das Ausgangssignal des Drehmomentsensors (3) empfängt und das fahrerinduzierte Lenkdrehmoment mißt;
eine Fahrzeuggeschwindigkeits-Meßeinrichtung (109b), die das Ausgangssignal des Fahrzeuggeschwindigkeitssensors (10) empfängt und die Fahrzeuggeschwindigkeit mißt;
einen Speicher (109e), der die der Rutschkupplung (14) zuzuführenden Stromwerte speichert, die von der Fahrzeuggeschwindigkeit und dem Lenkdrehmoment vorbestimmt sind;
eine Kupplungsstrom-Bestimmungseinrichtung (109f), die aus dem Speicher (109e) einen dem gemessenen Lenkdrehmoment und der gemessenen Fahrzeuggeschwindigkeit entsprechenden geeigneten Stromwert ausliest und den der Rutschkupplung (14) zuzuführenden Strom bestimmt;
eine Kupplungsstrom-Steuereinrichtung (109h), die den der Rutschkupplung (14) zuzuführenden Strom nach Maßgabe des Ausgangssignals der Kupplungsstrom-Bestimmungseinrichtung (109f) steuert.

6. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 2, wobei die Steuereinheit (109) umfaßt:
eine Lenkdrehmoment-Meßeinrichtung (109a), die das Ausgangssignal des Drehmomentsensors (3) empfängt und das fahrerinduzierte Lenkdrehmoment mißt;
eine Fahrzeuggeschwindigkeits-Meßeinrichtung (109b), die das Ausgangssignal des Fahrzeuggeschwindigkeitssensors (10) empfängt und die Fahrzeuggeschwindigkeit mißt;
einen Speicher (109c), der die an den Motor (13) anzulegenden Spannungswerte speichert, die von der Fahrzeuggeschwindigkeit und dem Lenkdrehmoment vorbestimmt sind;
eine Motorspannungs-Bestimmungseinrichtung (109d), die aus dem Speicher (109c) einen dem gemessenen Lenkdrehmoment und der gemessenen Fahrzeuggeschwindigkeit entsprechenden geeigneten Spannungswert ausliest und die an den Motor anzulegende Spannung bestimmt; und
eine Motorspannungs-Steuereinrichtung (109g), die die an den Motor angelegte Spannung nach Maßgabe des Ausgangssignals der Motorspannungs-Bestimmungseinrichtung (109d) steuert.

7. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 6, wobei die Steuereinheit (109) umfaßt:
einen zweiten Speicher (109e), der der Rutschkupplung (14) zuzuführende Stromwerte speichert, die von der Fahrzeuggeschwindigkeit und dem Lenkdrehmoment vorbestimmt sind;
eine Kupplungsstrom-Bestimmungseinrichtung (109f), die aus dem zweiten Speicher (109e) einen dem gemessenen Lenkdrehmoment und der gemessenen Fahrzeuggeschwindigkeit entsprechenden geeigneten Stromwert ausliest und den der Rutschkupplung zuzuführenden Strom bestimmt; und
eine Kupplungsstrom-Steuereinrichtung (109h), die den der Rutschkupplung (14) zugeführten Strom nach Maßgabe des Ausgangssignals der Kupplungsstrom-Bestimmungseinrichtung steuert.

8. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 5 oder 6, wobei der Speicher (109e) umfaßt:
einen Untersetzungsverhältnisspeicher (209c), der vorbestimmte Untersetzungsverhältnisse des Lenkdrehmoments relativ zur Fahrzeuggeschwindigkeit speichert; und
einen Steuerdrehmomentrechner (209d), der durch Multiplikation des gemessenen Lenkdrehmoments mit dem entsprechenden Untersetzungsverhältnis ein geeignetes Steuerdrehmoment errechnet, so daß die Kupplungsstrom-Bestimmungseinrichtung (109f) aufgrund des errechneten Steuerdrehmoments einen geeigneten Kupplungsstromwert bestimmt.

9. Motorgetriebenes Servolenksystem für ein Fahrzeug nach Anspruch 7, wobei der erste Speicher (109c) umfaßt:
einen ersten Untersetzungsverhältnisspeicher (209a), der erste vorbestimmte Untersetzungsverhältnisse des Lenkdrehmoments relativ zur Fahrzeuggeschwindigkeit speichert; und
einen ersten Steuerdrehmomentrechner (209b), der durch Multiplikation des gemessenen Lenkdrehmoments mit dem entsprechenden ersten Untersetzungsverhältnis ein erstes geeignetes Steuerdrehmoment errechnet, so daß die Motorspannungs-Bestimmungseinrichtung (109d) aufgrund des ersten errechneten Steuerdrehmoments einen geeigneten Motorspannungswert bestimmt;
und wobei der zweite Speicher (109e) umfaßt:
einen zweiten Untersetzungsverhältnisspeicher (209c), der zweite vorbestimmte Untersetzungsverhältnisse des Lenkdrehmoments relativ zur Fahrzeuggeschwindigkeit speichert; und
einen zweiten Steuerdrehmomentrechner (209d), der durch Multiplikation des gemessenen Lenkdrehmoments mit dem entsprechenden zweiten Untersetzungsverhältnis ein zweites geeignetes Steuerdrehmoment errechnet, so daß die Kupplungsstrom-Bestimmungseinrichtung (109f) aufgrund des zweiten errechneten Steuerdrehmoments einen geeigneten Kupplungsstromwert bestimmt.

10. Motorgetriebenes Servolenksystem nach einem der Ansprüche 2 bis 9, umfassend:
ein erstes Ritzel (5), das mit einer Lenkspindel (2) betriebsmäßig verbunden und über diese durch das Lenkrad (1) antreibbar ist;
eine Zahnstange (6), die durch Spurstangen (8a, 8b) mit einem Paar von lenkbaren Laufrädern betriebsmäßig verbunden ist und an der eine erste (6a) und eine zweite (6b) Verzahnung ausgebildet ist, wobei die erste Verzahnung mit dem ersten Ritzel kämmt;
ein zweites Ritzel (18), das mit der zweiten Verzahnung an der Zahnstange kämmt; wobei:
der Motor durch ein Untersetzungsgetriebe (R) mit dem zweiten Ritzel (18) betriebsmäßig verbunden ist, um die vom Fahrer über das Lenkrad, die Lenkspindel (2), das erste Ritzel (5) und die erste Verzahnung induzierte Lenkbewegung der Zahnstange mit einer Lenkhilfskraft zu unterstützen;
wobei die zwischen dem Motor und dem zweiten Ritzel (18) angeordnete Rutschkupplung (14) Kraft zwischen diesen im wesentlichen proportional zur Stärke des ihr zugeführten Stroms überträgt; und
die zwischen dem Motor und dem zweiten Ritzel (18) angeordnete Schaltkupplung (17) die betriebsmäßige Verbindung zwischen dem Motor und dem zweiten Ritzel (18) selektiv unterbricht, wenn die Fahrzeuggeschwindigkeit über den vorbestimmten Wert ansteigt, und die betriebsmäßige Verbindung dazwischen herstellt, wenn die Fahrzeuggeschwindigkeit unter den vorbestimmten Wert sinkt.

Drawing